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<head>
    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        let username = 'admin';
        let password = '123456';
        do{
            if( prompt('请输入用户名：') != username ||   prompt('请输入密码：') != password)
                alert('用户名或密码输入有误');
            else {
                alert('登录成功！');
                break;
             }
        }while(true)

    </script>
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